Single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and
Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ were studied using Transverse
field (TF)-$\mu$SR. We measured the penetration depth in the
superconducting state by fitting the $\mu$SR spectra to a
microscopic model for a range of applied magnetic fields and
dopings. We have fit the temperature dependence of the
superfluid density to a phenomenological two-gap model. We find
that the low-temperature superfluid density varies roughly
linearly with superconducting T$_C$, both of which decrease with
increasing Co substitution above x = 0.06.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2010.MAR.H39.4